The present invention relates to a photomask used for forming a fine pattern in fabrication of a semiconductor integrated circuit device and a pattern formation method using the photomask.
Recently, there are increasing demands for refinement of a circuit pattern due to a high degree of integration of a large scale integrated circuit device (hereinafter referred to as an LSI) realized by using a semiconductor. Therefore, in lithography technique employed for forming a desired pattern on a wafer, it has become difficult to form the pattern merely by reducing the exposing wavelength λ or increasing the numerical aperture NA, and as a result, various super-resolution techniques are now employed. As one of the super-resolution techniques, an attenuated (half-tone) phase-shifting mask that is comparatively easily fabricated and is minimally restricted in the shape of a pattern to be formed is widely employed.
FIG. 13A shows an exemplified cross-sectional structure of the attenuated phase-shifting mask. In the attenuated phase-shifting mask of FIG. 13A, a mask pattern is provided by forming a halftone portion 2 with transmittance of 5 through 12% against incident light on a transparent substrate 1 made of quartz or glass. The halftone portion 2 inverts the phase of the incident light by 180 degrees with respect to an opening 3 formed therein. In the mask pattern thus formed, the phase of light having passed through the opening 3 is different from the phase of light having passed through the halftone portion 2 by 180 degrees in the vicinity of an edge of the halftone portion 2, and hence, these lights cancel each other. As a result, the minimum value Imin of the light intensity obtained in the vicinity of the edge of the halftone portion 2 is lowered so that high contrast can be attained.
FIG. 13B is a diagram for showing a light intensity profile obtained by using the attenuated phase-shifting mask of FIG. 13A, and FIG. 13C is a schematic diagram for showing the plane structure of a pattern to be transferred onto a wafer when the opening 3 of FIG. 13A is a hole pattern. As shown in FIG. 13B, the highest light intensity appears at the center of the opening 3. Also, an unwanted side lobe with the local maximum value of the light intensity is caused around the opening 3 due to the influence of diffraction of the light caused at the edge of the halftone portion 2. The local maximum value of the light intensity of the side lobe is in proportion to the dimension of the opening 3.
FIG. 13D shows another exemplified cross-sectional structure of the attenuated phase-shifting mask. In FIG. 13D, like reference numerals are used to refer to like elements used in the mask shown in FIG. 13A so as to avoid repetition of the description. The attenuated phase-shifting mask of FIG. 13D has a larger opening 3 than the attenuated phase-shifting mask of FIG. 13A.
FIG. 13E is a diagram for showing a light intensity profile obtained by using the attenuated phase-shifting mask of FIG. 13D, and FIG. 13F is a schematic diagram for showing the plane structure of a pattern to be transferred onto a wafer when the opening 3 of FIG. 13D is a hole pattern. As shown in FIG. 13E, when the opening 3 is increased in the dimension, the local maximum value of the light intensity of the side lobe exceeds an exposure threshold value (i.e., the minimum value of the light intensity for sensitizing a resist to be exposed). As a result, an unwanted pattern (a side lobe pattern) is disadvantageously transferred onto the wafer as shown in FIG. 13F.
As a method for preventing the occurrence of a side lobe, mask bias (modification of a mask pattern caused by proximity correction or the like) is increased and exposure is reduced. In this method, however, the contrast is disadvantageously lowered. Accordingly, in a mask pattern having an opening with a dimension not more than 0.5×λ/NA (wherein λ is a wavelength of exposing light and NA is a numerical aperture of a reduction projecting optical system of an aligner; which are also applied in description below) difficult to attain sufficient contrast, the maximum exposure and the minimum mask bias not causing a side lobe is employed for attaining higher contrast. Alternatively, in a mask pattern having both a first opening with a first dimension not more than 0.5×λ/NA and a second opening with a second dimension larger than 0.5×λ/NA, the exposure is set to one for attaining high contrast for the first opening with the first dimension. In such a case, however, since the local maximum value of the light intensity of a side lobe is in proportion to the dimension of an opening as described above, the local maximum value of the light intensity of the side lobe exceeds the exposure threshold value with respect to the second opening with the second dimension.
In order to overcome this problem, Patent Document 1 (Japanese Laid-Open Patent Publication No. 9-281690) discloses a method in which a second opening is formed in a light-shielding region made of a light-shielding portion for preventing the occurrence of a side lobe in the second opening. FIG. 14 is a plan view of a conventional photomask disclosed in Patent Document 1. As shown in FIG. 14, a halftone portion 2 is provided at the center of a transparent substrate (not shown), and a light-shielding portion 4 is provided in the periphery of the transparent substrate. The halftone portion 2 is provided with first openings 3A each having a first dimension not more than 0.5×λ/NA, and the light shielding portion 4 is provided with second openings 3B each having a second dimension larger than 0.5×λ/NA.